Heretofore, carbon black has been used as a reinforcing filler for rubber because carbon black provides higher reinforcement and more excellent abrasion resistance than other fillers. Recently, because of social requirements to save energy and to save resources, particularly to cut down fuel consumption of automobiles, a decrease in the heat buildup of rubber compositions is also required.
For decreasing the heat buildup of a rubber composition by using carbon black, use of a small amount of carbon black or use of a carbon black having a large particle size is considered. However, it is well known that, in both methods, decreasing heat buildup is in a contradictory relation with improving reinforcement and abrasion resistance of a rubber composition.
On the other hand, silica is known as a filler which provides decreased heat buildup of a rubber composition, and applications for many patents, for example, Japanese Patent Application Laid-Open No. Hei-3-252431, have heretofore been made.
However, silica has a tendency to form agglomerates of particles by hydrogen bonding of a silanol group which is a functional group on the surface. For improving the dispersion of silica particles into rubber, the mixing time must be increased. When dispersion of silica particles into rubber is insufficient, a problem arises in that processability in processes such as extrusion and the like deteriorates due to the increase in the Mooney viscosity.
Moreover, the surfaces of the silica particles are acidic. Therefore, there are problems in that, during vulcanization of the rubber composition, basic substances used as vulcanization accelerators are adsorbed such that vulcanization is not carried out sufficiently, and a sufficient modulus of elasticity is not obtained.
In order to solve these problems, various types of silane coupling agents have been developed. For example, use of a silane coupling agent as a reinforcing material is described in Japanese Patent Application Publication No.Sho-50-29741. However, the use of a silane coupling agent as a reinforcing material is still insufficient for improving fracture properties, workability, and processability of a rubber composition to high standards. Rubber compositions in which a combination of silica and a silane coupling agent is used as a reinforcing material are described in Japanese Patent Application Publication No. Sho-51-20208 and others. However, this method of using a combination of silica and a silane coupling agent as a reinforcing material has a drawback in that flow of the uncured rubber composition is markedly inferior and workability and processability deteriorate, although reinforcement of the rubber composition can be remarkably improved and the fracture properties are improved.
The drawbacks of the conventional technologies in which silane coupling agents are used arise due to the following mechanism. When the mixing temperature of rubber is low, the silanol group on the surface of the silica does not react sufficiently with the silane coupling agent, and as a result, the reinforcing effect is not obtained. Dispersion of the silica into the rubber is also inferior, and this causes deterioration of the low heat buildup property which is the strong point of a rubber composition containing silica. Moreover, some of the alcohol formed by the reaction of the silanol group on the surface of the silica and the silane coupling agent does not vaporize completely (during mixing) because of the low mixing temperature, and the residual alcohol in the rubber vaporizes during an extrusion process so as to form blisters.
On the other hand, when mixing is conducted at high temperatures of 150.degree. C. or higher, the silanol group on the surface of the silica and the silane coupling agent sufficiently react with each other, and as a result, the reinforcing property is improved. Because dispersion of the silica into the rubber is also improved, a rubber composition having a good low heat buildup property is obtained, and the formation of blisters in an extrusion process is suppressed. However, in this temperature range, gelation of the polymer caused by the silane coupling agent takes place simultaneously, and the Mooney viscosity is markedly increased. Thus, processing in later stages becomes impossible in actuality.
Therefore, when a silane coupling agent is used in combination with silica, a multistep mixing must be conducted at a temperature lower than 150.degree. C., and marked decrease in productivity is inevitable.